1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
In recent years, from a viewpoint of compactness, low in cost, and concern for the environment, a demand for an image forming apparatus which consumes less energy and produces less waste toner has increased. Compared with a conventional laser optical system, a light-emitting diode (LED) exposure system has a short optical path and contributes to a miniaturization of a main body of the system. Further, it does not need a polygon motor, so that energy consumption can be reduced. At present, however, LEDs need to be located close to an object to be exposed on account of light quantity. Therefore, a close proximity exposure system is adopted. In an ordinary electrophotographic operation, a surface of a photosensitive member is electrostatically charged and exposed to form a latent image, and after the latent image on the photosensitive member is visualized as a developed image by development processing, the developed image is transferred to paper in transfer processing. A residual transfer toner on the photosensitive member is removed by a cleaning (CLN) mechanism, and the procedure is repeated from the charging processing. In this case, the residual transfer toner becomes waste toner.
On the other hand, a so-called CLN-less system has been discussed which recovers the residual transfer toner without using the CLN mechanism while forming a developed image in the development processing. The CLN-less system is an energy-saving technology, which can realize waste toner less image formation and reduce drive power consumption because this system does not use the CLN mechanism which has been a main cause of a drive torque of the photosensitive member.
However, for example, Japanese Patent Application Laid-Open No. 11-184216 discusses that in a case where a proximity exposure system is combined with a CLN-less system, since toner exists and passes through the photosensitive member in an exposure unit, the toner adheres to the exposure unit, resulting in defective images. If an electric charge amount of the toner passing through the exposure unit is small, the toner is likely to scatter and adhere to the optical system, so that occurrence of defective images with density non-uniformity may increase. To reduce this phenomenon, it is necessary to increase a charge amount of residual transfer toner before the exposure unit or before charging.
U.S. Pat. No. 7,194,226 discusses providing a developer charge amount control unit to charge a residual transfer toner after transfer processing and before charging processing, and also discusses necessity to achieve a balance between prevention of contamination of a charging member and electric potential unevenness after charging by the developer charge amount control unit. More specifically, it can be understood that when the charging member is prevented from being contaminated by increasing the charge amount of the residual transfer toner with using the developer charge amount control unit, there is a limit to the charge amount of the residual transfer toner that can be increased considering charging unevenness.
Meanwhile, a technique which uses a high coloring toner can save energy since it can reduce toner consumption required to obtain a same density, and also reduce electric power needed to fix a toner image. When a toner amount required to obtain a maximum density is reduced, another advantage can be obtained that a decrease in the charge amount of the residual transfer toner in the transfer unit can be reduced.
When a proximity exposure system is used combined with a CLN-less system, since a toner exists on the photosensitive member and passes through the exposure unit, in order to prevent the toner from adhering to the exposure unit and generating defective images, the charge amount of the residual transfer toner before exposure processing or charging processing needs to be increased. When the toner is prevented from adhering to the exposure unit by providing a developer charge amount control unit between after the transfer processing and before charging processing, the system needs to be enlarged, resulting in an increase in cost. Since there is a limit to increase the charge amount of the residual transfer toner, there are not only contamination of the charging member but also adhesion of toner to the exposure unit and effects of preventing or reducing the generation of defective images are insufficient.
Further, the charge amount of the residual transfer toner can be increased by increasing an absolute value of an average charge amount of a developed image on the photosensitive member. However, if the average charge amount of a developed image is set at as high as not less than 50 μC/g and not higher than 90 μC/g, the developed image cannot be readily transferred from the photosensitive member to paper in the transfer processing.